During hydraulic fracturing pumping operations on oil or gas wells it is often necessary to pump fluids downhole from the surface at high pressures and flow rates. Depending on the well and the particular operation being performed, it may be necessary to provide in excess of 12000 HP to overcome the downhole pressure at the desired volumetric rate.
In a typical fracturing pump setup, several pumps are used, each typically being fitted with a diesel engine as primary driver for a high pressure triplex or quintuplex positive displacement pump, coupled through a five to seven speed transmission. Each pump typically delivers between 1500 to 2000 HP depending on the type and size of the engine fitted. Large fracturing operations can require more than 16 pumping units to be available at the well site to meet the pumping power requirements and provide backup in case of failure of individual units.
Due to the nature of the wells it is often necessary to start the pumping operation against an existing well pressure. This is known to put a high stress on the pump transmission and occasionally cause failure.
In the event that an active pump fails during the operation, a stand-by unit is required to take its place. In this case the stand-by pump will need to start against a high pressure from standstill. In order to do this, operators must put the engine at speed and engage the desired gear. The transmissions typically used are fitted with a torque converter that allows a limited amount of slip between engine speed and pump speed. However the unit will only tolerate running with such internal slippage for a very limited amount of time.
An increase is sometimes required to the overall pumping volumetric rate. In most cases this can only be done by changing gears on the transmission of some of the pumps, or starting other pumps.
All of the above operations place a great strain on the transmission systems, pumps and engines which can lead to unreliability and reduced operational life.
Conventional cementing or fracturing pump units are typically provided with a five-gear automatic transmission to allow either the full pump flow capacity or the full pump input torque. The weakness of this system is that for pumping under pressure, the engine must be at peak torque speed when the transmission is shifting up or down damaging the transmission disks.
An alternative to this type of transmission is a conventional hydrostatic transmission (HST) comprising a variable volume pump drive by the engine and a hydraulic motor drive by this pump. Such systems provide a gearless drive mechanism that translates engine power to hydraulic power continuously. Such systems are widely applied in construction machinery, because of their superior speed changeability. However HST does have certain problems that make is unsuitable for pumping systems, e.g. limited low speed/torque range and narrow shift range.
HST is one form of a continuously variable transmission (CVT). A CVT avoids some of the problems of changing gears in a conventional automatic gear box by allowing any ratio of input and output to be obtained. Mechanical CVTs are well known in automotive applications.
An extension to CVT design, sometimes known as the Infinitely Variable Transmission (IVT), allows the transmission to drive a vehicle backwards as well as forwards. Transmission input is split into two shafts with one connected to an epicyclic gear set and the other to a CVT. The output from the CVT is connected to a different set of gears in the epicyclic. The gear that does not draw power from engine or CVT transfers torque to the transmission output. The epicyclic gear set acts as a mechanical adding machine to subtract or add one speed from the other.
Hybrid automobiles also use epicylic gear system to allow both power sources to be connected to the vehicle transmission.
An object of this invention is to provide a transmission system that has the advantages of a CVT in a form that can be used effectively in a pump drive system. This object is achieved in the invention by the use of a epicylic gear system.